1. Field of the Invention
The present invention relates to the drilling of well bores, well completion methods, and the extraction and/or utilization of thermal energy from rock formations beneath the surface of the earth.
2. History of Related Art
Permeable geologic strata having high temperatures are found in numerous site-specific locations around the globe. When meteoric water percolates down into these formations, the water is heated and may flow to the surface as geysers and hot springs. Impermeable geologic rock formations, typically Precambrian rocks, having high temperatures are found almost everywhere around the globe and are generally located at deeper depths than high temperature permeable geologic strata which is typically sedimentary rock in nature. These impermeable Precambrian formations are generally considered dry and heat may be recovered from these formations by means of the hot dry rock (HDR) geothermal production process in which water is pumped down a well drilled into these deep hot impermeable rock formations and heated by contact with the rock. If the rock in its natural state does not have a sufficient network of cracks and fissures for the water to flow through to pick up heat, as is the usual case, the rock is hydraulically fractured to produce such a fracture network by means of fluid pressure. Various means to continuously circulate the heat from these HDR formations have been established.
Today, energy is supplied primarily by fossil fuels such as coal, oil, and gas. These resources are finite and are expected so to be in short supply in the readily foreseeable future. Also, the use of fossil fuels appears to cause serious environmental problems. Further, the United States currently imports a large percentage of its oil. Dependence on foreign oil is increasing as domestic reserves diminish. Thus, development of alternative sources of energy is necessary. When coal is burned, significant amounts of sulfur and nitrogen oxides are released to the atmosphere. These gases combine with water in the atmosphere to produce acids, which are brought to earth by rainfall downwind of the emissions source. This “acid rain” has a deleterious effect on aquatic and plant life. On a mere long-range scale, the atmosphere may be warming because of the “greenhouse effect” which may be caused by large quantities of carbon dioxide being released to the atmosphere as a result of burning of fossil as fuels. The long-term consequences of the greenhouse effect are currently a matter of debate; they may include melting of the polar ice caps, with the resultant increase in sea level and flooding of coastal cities, and increased desertification of the planet. Evidence pointing toward greenhouse effect warming includes increases in the carbon dioxide, content of the atmosphere over the past century and weather records that seem to indicate an upward trend in atmospheric temperatures. These facts point to the need, to consider mitigating action now, before we are overtaken by our own emissions.
Hydropower, the world's primary non-fossil energy source, is both inexpensive and clean. Hydropower has been widely developed in many parts of the world, but will never fill more than a small part of the world's total energy needs. Other alternative energy sources are nuclear fission, solar, wind, fusion, and geothermal. Nuclear fission is already widely used, but is currently suffering from a lack of public confidence, particularly in the United States, as the result of common knowledge of such incidents as Three Mile Island and Chernobyl. There are few nuclear power plants currently in the planning or construction stages. Solar power has been demonstrated on a small scale, as has wind power. Although both of these are renewable energy sources, they are subject to the whims of local weather conditions and can be relied upon to deliver power only intermittently. Nuclear fusion is, potentially, an almost unlimited source of energy, relying for fuel upon isotopes of hydrogen, which are found in abundant amounts in seawater. However, fusion has been unambiguously demonstrated only in the highly intractable form of a thermonuclear explosion. Decades may pass before ignition and containment of a fusion reaction by controllable, non-nuclear ignition sources, such as lasers, will be developed to the point where nuclear fusion may find practical application as a power source.
Geothermal resources, in the form of naturally occurring hydrothermal fluid systems, are being exploited today to provide useful energy as electrical power or heat in many parts of the world. At present, hydrothermal sources provide only a minute fraction of the world's energy needs, though the potential resource base available for exploitation is of the same order of magnitude as fossil fuel resources. Hydrothermal resources are much cleaner than fossil fuels with regard to greenhouse gas emissions, generally releasing only about 10 percent or less of the amount of carbon dioxide produced by burning an energy-equivalent amount of fossil fuel. However, hydrothermal resources are of limited geographical extent, occurring primarily in areas of tectonic or volcanic activity. Thus, many densely inhabited parts of the world are poorly located for the exploitation of hydrothermal sources.
Hot Dry Rocks (HDR), typically Precambrian rocks, underlie much of the globe. Unlike hydrothermal resources, HDR is widely distributed about the earth, generally underlying the sedimentary based hydrothermal formations. The HDR resource potential is a resource of vast magnitude and, like fusion, HDR can provide an almost unlimited source of energy for the planet. Hydrothermal plants now in operation demonstrate conclusively that the heat of the earth can be used as a practical source of both thermal and electrical energy. The HDR process is a logical extension of hydrothermal technology to tap into a vastly larger and universally distributed energy resource.
The conventional teaching of extracting energy from HDR involves creation of a closed liquid circulation system comprised of an HDR reservoir and the above-ground equipment. Initially, an injection well is drilled into hot dry rock and hydraulic fracturing techniques are used to induce permeability by stimulating existing natural joints or creating new fractures. Hydraulic stimulation and fracturing are widely used in petroleum recovery. An HDR reservoir is thus created, the size of which is governed by the pressure, rate and volume of the hydraulic fracturing fluid applied to the rock, the nature of the rock structure, and in situ stresses as have been clearly demonstrated in modern HDR completions such as those cited in Geodynamics Limited Quarterly Report period ending Mar. 31, 2004. Additional wells are subsequently drilled to provide the rest of the fluid circuitry necessary for establishing the closed loop circulation system. To produce heat production, liquid is pumped down the injection well, heated by the hot rock of the HDR reservoir, and recovered from a second well, a production well, drilled into the reservoir at some distance from the injection well. Multiple injection and recovery wells may be used within the basic closed loop circulation system. Heat exchangers at the surface are used to recover the heat from the water for use in electric power generation or for direct thermal applications. The water is then re-injected into the HDR reservoir via the injection well. In this manner, heat can be continuously mined from otherwise inaccessible geothermal sources. Essentially no venting of gaseous or saline fluids to the environment occurs. Thus, the HDR process does not emit carbon dioxide or acid rain precursors, such as sulfur dioxide, and is in the same class as solar, wind, or hydro-power in being an environmentally benign source of energy. The primary application of water heated in an HDR reservoir will be to generate steam or to vaporize another working fluid, such as ammonia or isobutane, for use in producing electric power.
U.S. Pat. No. 3,786,858, issued Jan. 22, 1974, describes the HDR process. A publication issued by the Los Alamos National Laboratory in July, 1989 which is designated LA-1 15 14-MS and entitled “Hot Dry Rock Geothermal Energy a New Energy Agenda for the 21st Century, describes a number of concepts for use of HDR energy. There are experimental HDR sites in Europe, Japan, the U.S. and commercial HDR ventures in the process of being developed in Europe and Australia. The Geothermal Resources Council periodically publishes a bulletin dealing with geothermal energy matters. The SPE Paper No. 30738 titled—“Hot Dry Rock: A versatile Alternative Energy Technology” by D. V. Duchane, Earth and Environmental Sciences Div., Los Alamos National Laboratory, presented October 1995 describes the current state of the HDR development.
The public offering prospectus offered by Geodynamics Limited of Australia, entitled “Geodynamics Limited—ABN 55 095 006 090—Power from the Earth—Prospectus” dated Aug. 13, 2002 provides the most modern thought process and effort to develop and commercialize a HDR electrical generation system. The Geodynamics HDR model provides for multiple “lens” of opened natural rock joint groups to be vertically interconnected through common injection and production well bores to provide the basis from which to mine heat from a “triplet” of wells. The heat is mined from the reservoir rock through continuous circulation from an injector well to multiple production wells that provide a pressure sink in order to induce directional circulation. This commonly-known configuration provides a point-to-point directionally-specific pressure-sink-type closed-loop circulation system.
Companies which provide electric power must have sufficient power generating capacity to not only meet base load demand but also must meet peak demand, or maximum demand, which usually occurs in the late afternoon of a hot summer day. Power production apparatus which is in reserve must be capable of being brought on-line very quickly, in order to prevent “brown-outs” or load shedding. Load shedding refers to cutting off power to some users in order to avoid catastrophic shut-down of the entire system. Such apparatus is commonly termed “spinning reserve”. Spinning reserve power, or peaking power, is costly because the equipment used to generate spinning reserve power is in revenue-generating use only a portion of the time rather than 24 hours a day. Also, the equipment is generally more expensive to purchase and operate than base-load electric power production equipment.
U.S. Pat. No. 5,685,362, issued Nov. 11, 1997 describes a method for meeting peak power demands with a HDR heat mining system and a power generating plant. Thus, the U.S. Pat. No. 5,685,362 invention effectuates use of an HDR power generation system for electric load following. The U.S. Pat. No. 5,685,362 invention may also be termed on-demand power peaking. Peaking power from an HDR system would be cheaper to generate than peaking power from other sources yet can be sold at the same price as peaking power generated by other means, such as a gas turbine. Use of an HDR system in a load-following mode rather than just to provide base-load power will reduce the total cost of operation of an HDR system. The incremental cost of equipment to operate in peaking mode is expected to be modest. This process is described in an undated paper titled “The Geothermal Analog of Pumped Storage for Electrical Demand Load Following” by Donald W. Brown, Los Alamos National Laboratories, Earth and Science Division, Los Alamos, N. Mex. 87545. The invention U.S. Pat. No. 5,685,362 invention teaches the practice of heat mining by continuous fluid circulation through an injection and multiple production wells coupled with the method of periodic reduction of the production well back pressure to allow a short term flow of a greater volume than the steady state flow volume to be produced thereby providing periodic “peaking” power capacity to provide electrical generation load following characteristics.
The gasification of organic material under supercritical water conditions as taught by Modell et al in U.S. Pat. No. 4,113,446 issued Sep. 12, 1978, titled: Gasification Process, is known in the art. Also the use of a subterranean well bore for the purpose of providing a gravity based reactor vessel from which to perform continuous supercritical water chemical reactions as taught by Titmas in U.S. Pat. No. 4,594,164, issued Jun. 10, 1986, titled: “Method and Apparatus for Conducting Chemical Reactions at Supercritical Conditions”, is exemplary of the state of the art that is also known. These teachings provide a process of conversion of organic material by way of supercritical water anaerobic gasification. Oil and gas resources are a finite resource whose production capacity is rapidly declining and it is therefore essential that the organic carbon found in coal that is found in vast quantities on a world wide basis become useful through the ability to convert coal to clean burning fuel gasses and liquids while capturing the various other marketable or harmful constituents for useful sale or disposal as the case may be.
The HDR concept of generating geothermal heat has been know for many decades and has generally been relegated to a non-commercial technology due to the prohibitively high cost of drilling multiple wells into the deeply buried crystalline type Precambrian hot dry rock formations. Modern attempts to commercialize the HDR method of generating geothermal energy have to locate a very unique set of conditions in geologic areas that exhibit exceptionally high geothermal gradients to provide manageable project drilling costs vis-a-vis relatively shallow drilling depths. Typically, these developments seek a site that has significant sedimentary overburden before drilling into the Precambrian formations to access the HDR thus being minimizing drilling costs by drilling a minimal section of the well bore in the Precambrian type rock. Further, these modern attempts to commercialize the HDR geothermal production are economically restricted by the high cost of drilling injection and multiple production wells. The high cost well bores severely constrain the project design from being designed as an optimal production system to mine the maximum heat available in the source rock.
The present invention provides a method of drilling, completing and producing a geothermal reservoir in order to a) economically locate said geothermal reservoirs in most all areas of the world, even those areas with lower thermal gradients that are currently uneconomical to produce, b) economically locate said geothermal reservoirs at depths that provide supercritical water conditions, c) maximize the effective recovery of geothermal heat, per unit volume of HDR formation and d) provide a method of producing and utilizing said geothermal heat energy for individual or simultaneous direct and/or indirect applications such as any individual or combination of the generation and use of high temperature geothermal process steam, the generation and use of geothermal heat energy for the production of electricity and/or the generation and use of geothermal heat energy in the processing of organic carbon or other chemical reactions.